[Below is the original script. But a few changes may have been made during the recording of this audio podcast.]

It's no secret that there are more and more of us every day. From fewer than a billion 200 years ago to more than 6.6 billion people on the planet today. And the U.N. expects more than nine billion by mid-century.

This kind of exponential population growth has consequences for the planet, from stretched natural resources, such as fresh water supplies, to burgeoning levels of greenhouse gases in the atmosphere.

By their calculations, every American child born today will add roughly 9,441 metric tons of carbon dioxide to the atmosphere under current conditions. For comparison, a Chinese child would add roughly one fifth that amount while a boy born in Bangladesh will add 1/160th.

Having one fewer child would reduce a family's climate burden 20 times more than driving a fuel-efficient car or using energy-saving appliances, according to this statistical analysis. That doesn't seem to be stopping anyone in developed countries like the U.S. New research published in Nature this week shows that birth rates in the most developed countries are rising again.

Of course, who can judge the right number for human population? But a little family planning might be the best measure of an environmentalist and another baby boom could be bad news for the planet.